1. Field of the Invention
The present invention relates to a medical liquid infusion apparatus to infuse a medical liquid held in a medical liquid container into a patient.
2. Description of the Related Art
Generally, there has been known a medical liquid infusion apparatus provided with a driving source and configured to discharge a medical liquid inside a medical liquid container using a driving force of the driving source so as to infuse the medical liquid into a patient (for example, Patent Document 1 : JP-A-9-262288).
This medical liquid infusion apparatus uses a compressed gas cylinder filled with a liquefied carbon dioxide as the driving source. The medical liquid infusion apparatus is configured in such a manner that the interior of a compression chamber allowed to communicate with the compressed gas cylinder is brought into an under-pressure condition resulting from a saturated vapor pressure of the liquefied carbon dioxide so as to expand a bellows container that defines the compression chamber, and a pressure is applied to a medical liquid cartridge by the bellows container for the medical liquid to be discharged.
Because the medical liquid infusion apparatus uses a compact compressed gas cylinder that does not need power supply as the driving source, it is possible to achieve a configuration suitable for a portable use by the patient.
The medical liquid infusion apparatus of Patent Document 1 described above uses a pressure resulting from a saturated vapor pressure of the liquefied carbon dioxide as the driving source, and therefore has a characteristic that the driving force varies markedly when an environmental temperature rises above the environmental temperature at which the medical liquid is discharged at a predetermined flow rate (hereinafter, referred to as the reference operation temperature).
More specifically, because the saturated vapor pressure has a property that it increases in a quadratic curve with a rise in temperature (see L2 of FIG. 6), for example, a comparison between a change in vapor pressure when the environmental temperature dropped from 25° C. to 20° C. and a change in vapor pressure when the environmental temperature rose from 25° C. to 30° C. reveals that although a difference in temperature is the same (5° C.), a change in vapor pressure with a rise in temperature is far larger.
The medical liquid infusion apparatus of Patent Document 1 therefore has a markedly large change in flow rate when an environmental temperature rises above the reference operation temperature. Generally, an allowable range is set for the predetermined flow rate. However, in order to set an allowable range for the reference operation temperature in the medical liquid infusion apparatus of Patent Document 1 for the actual flow rate to fall within the allowable range, the allowable range has to be set smaller on the plus side and larger on the minus side.
The medical liquid infusion apparatus, however, is assumed that it is used in a case where it is mounted on the bedside of the patient or the like (that is, a case used under the room temperature condition) and in a case where it is used while being carried with the patient (that is, a case used under the thermal environment close to the body temperature of the patient). Meanwhile, the reference operation temperature is often set to room temperature at which the management by the medical staff or the like becomes easy. Accordingly, in the case of a portable use by the patient, an environmental temperature increases from the reference operation temperature.
In view of the foregoing, there is a request to set the allowable range of the reference operation temperature on the plus side to the fullest extent possible.